1. Field of the Invention
This invention relates to radial ply tires or high performance tire compounds with higher modulus for hard apex, chafer (rim flange) and tread compound for improved handling and uniformity.
In the recent development of radial ply tires or high performance tires, the requirements for tire materials have become more and more specialized and diversified to achieve specific dynamic properties. For example, certain commercial superfiller radial ply and high performance tires have hard chafer and an apex with Shore A Hardness greater than 90 for improved ride and handling, which is achieved by compounds highly filled with carbon black to achieve high hardness. However, if there is too much high structure black in a compound, processing, i.e., mixing and extruding will be more difficult due to the higher Mooney Viscosity resulting from the good dispersion of carbon black.
With high performance tread, traction is most important in order to obtain safe and better handling tires even though the tread wear becomes poorer. In order to improve handling, a higher cornering coefficient is required. Higher cornering coefficient tires can be obtained from either a higher dynamic modulus tread or a lower angle of steel belts. Lower angle of steel belts may not be a good choice because of irregular wear problems and a harsher ride. Therefore, the better choice is to develop higher dynamic modulus tread to provide higher cornering coefficient for better handling. Higher dynamic modulus tread compounds can be achieved with an excessive amount of high structure, super abrasion furnace carbon black and lower oil. However, this lower oil, higher filler compound is extremely difficult to mix and extrude even though the compound provides better wear resistance.
Thus, it would be desirable to possess a tire with higher modulus for hard apex, chafer and tread compound for improved handling and uniformity as well as reduced problems of short scorch times during cure.
2. Description of Related Art
The use of hardening phenolic resin to meet tire manufacturers' requirements to achieve higher hardness compounds is known. It is necessary, however, to incorporate a curative to vulcanize the hardening resin because the unvulcanized hardening phenolic resin becomes soft when the temperature reaches its melting or softening point. A known curative system is a blend of phenolic resin with hexamethylenetetramine. For example, Chemical Abstracts 95:26446m (1981) discloses natural and synthetic rubbers used for tire bead reinforced with filler compositions comprising novolac resin and a hexamethylenetetramine cross-linking agent. Another example is Chemical Abstracts 100:69670u (1984) in which hard rubber products are prepared by vulcanizing compositions comprising diene rubber, cashew nutshell liquid-modified cresol-formaldehyde resins, and hexamethylenetetramine as the curing agent.
Hexamethylenetetramine, however, has had a toxicology problem in the tire industry, and many tire plants prohibit its use. Also, the preblend of phenolic resin with curatives causes problems of short scorch times when the preblended resins used with the masterbatch are discharged at high temperatures from an internal mixer.
U.S. Pat. Nos. 3,256,137, 3,266,970 and 3,281,311 all disclose the use of 1-aza-5-methylol-3,7-di-oxabicyclo (3,3,0) octane as an adhesive between rubber and reinforcing textile materials. None of those references, however, suggests the use of that compound as a resin curing agent as in the present invention.